The present invention relates to a phase change type optical recording medium, and its fabrication method.
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for rewriting or overwriting. One typical rewritable optical recording medium is of the phase change type wherein the recording layer is irradiated with laser beam to change its crystallographic state whereupon a change of reflectance by the crystallographic change is detected for reading. Optical recording media of the phase change type are of great interest since they can be overwritten by modulating the intensity of a single light beam and the optical system of the drive unit used for their operation is simpler as compared with magneto-optical recording media.
Most optical recording media of the phase change type use Ge--Te base materials which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. In recent years, it has been proposed to use compounds known as chalcopyrites.
Chalcopyrite compounds are extensively investigated as compound semiconductor materials, and applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described in Physics, Vol. 8, No. 8 (1987), page 441, Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), page 228, and other literature.
Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with antimony or Sb or bismuth or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See JP-A's 3-240590, 3-99884, 3-82593, 3-73384 and 4-151286.
In addition to these phase change type optical recording media using chalcopyrite compounds, JP-A's 4-267192, 4-232779 and 6-166268 disclose phase change type optical recording media wherein an AgSbTe.sub.2 phase forms upon the crystallization of a recording layer.
When information is recorded on a phase change type optical recording medium, the recording layer is irradiated with laser beam of power (recording power) high enough to bring the recording layer to a temperature higher than the melting point thereof. The recording layer is melted at spots with the recording power applied thereon, and then quickly cooled so that recorded marks of amorphous nature can be formed. When the recorded marks are erased, on the other hand, the recording layer is irradiated with laser beam having such a relatively low power (erasing power) as to bring the temperature of the recording layer to a temperature higher than that the crystallization temperature thereof but lower than the melting point thereof. The recorded marks with the erasing power applied thereon go back to the amorphous state because they are slowly cooled down after heated to the temperature higher than their crystallization temperature.
In the phase change type optical recording medium, dielectric layers are formed on both sides of the recording layer. Requirements for these dielectric layers are that:
(1) they can protect the recording layer or substrate against thermal hysteresis due to irradiation with laser beam,
(2) they can amplify reading signals making use of the interference effect of light reflected at the interface of each layer, and
(3) their thermal conductivity, etc. can be properly regulated to regulate record/erase characteristics.
For a dielectric layer meeting such requirements, a dielectric layer composed mainly of ZnS having a high refractive index is often used. For instance, JP-A 63-103453 discloses an optical information recording medium having a dielectric layer comprising a mixture of ZnS and SiO.sub.2. The publication states that among the advantages of the invention there are an increase in the sensitivity to incident light power upon recording, and an increase in the number of repetition of write/read cycles to which the dielectric material is exposed, and goes on that the sensitivity increase is due to the optimization of the heat constant of the dielectric layer, and the increase in the number of repetition of write/read cycles is due to the prevention of denaturation of the dielectric layer. JP-A 4-114330, too, discloses an optical information recording medium having a dielectric layer comprising a mixture of a compound selected from the group consisting of ZnS, etc., and a compound selected from the group consisting of Ta.sub.2 O.sub.5, etc. The publication alleges that among the advantages of the invention there are excellent humidity resistance, less defects, and no interlaminar separation.
According to experiments conducted by the inventors, however, it has been found that when a ZnS-containing dielectric material is used for the dielectric layers of the phase change type optical medium, repeated overwriting gives rise to some considerable jitters, which in turn bring about a considerable reduction in the number of repetition of write/read cycles. According to TDS analysis of a ZnS--SiO.sub.2 dielectric layer, it has also been found that, upon included in the form of a simple substance or dimer in the dielectric layer during its formation, sulfur is isolated and sublimed from the dielectric layer by heating to about 400.degree. C. This temperature is close to the melting point of the recording layer. Thus, the sulfur is diffused from within the dielectric layer into the recording layer upon recording, resulting in a change in the composition of the recording layer. Such actions lead to a decrease in the rate of erasure or fluctuations in the distance between recorded marks, which are believed to be attributable to the increased jitters due to repeated overwriting.